To reduce the quantity of atmospheric pollutants in exhaust gases from internal combustion engines and to meet the increasingly stringent regulations regarding the content of such gases, various methods and associated apparatus have been developed to purify exhaust gases. One such method is to recycle a portion of the exhaust gas from an engine back through the engine. By returning the exhaust gases, which consist essentially of inert gases, to the engine intake and mixing the returned exhaust gases with the air and/or air-fuel mixture being taken in by the engine, the nitric oxide content of the gases ultimately exhausted from the engine is decreased. In particular, the recycling of exhaust gases decreases the peak combustion temperatures in the combustion chambers of the engine so that the emission of nitric oxides, which is a function of such combustion temperatures, can be significantly reduced.
Particular apparatus used to practice the recycling method of emission control is described and discussed in paper No. 720123 published by the Society of Automotive Engineers, Inc. (SAE). The paper is entitled "Exhaust Recirculation and Spark Control -- A Speed Governed and Vacuum Modulated System" and is authored by E. J. Martin and D. R. Vance. Each version of the apparatus described in SAE paper No. 720123 includes a conduit extending from an engine exhaust line to a point in the engine air/fuel intake system. A valve in the conduit regulates the flow of exhaust gas from the exhaust line to the air/fuel intake system. The regulating valve is either a flap type or a poppet type valve.
Regulating valves have also been proposed that utilize rotatable, perforated plates, rather than flaps or axially movable valve bodies, as exemplified by the valves described and illustrated in copending, commonly owned application Ser. No. 429,625, filed Dec. 28, 1973.
Apparatus for practicing the recycling method of emission control is also described and illustrated in German Offenlegungsschrift No. 2,002,164 and in corresponding U.S. Pat. No. 3,675,633 issued to Nakajima et al. In the apparatus of the Nakajima patent, a recirculation control valve for controlling the quantity of exhaust gas being recycled through the engine has a construction similar to a conventional cam operated valve for the intake and exhaust of a cylinder of an internal combustion engine. The disc-shaped valve body of the recirculation valve is mounted on a valve stem that is moved by a rotatable cam. The cam is connected by a tie rod linkage to the throttle valve for the engine.
Since the stem of the Nakajima et al. valve must pass through the valve housing to engage the cam, the valve must be sealed against exhaust gas leakage along the interface between the valve stem and the inner surfaces of the valve housing. If, as happens during normal operations, the valve stem and the guide slot that receives the valve stem in the valve housing are exposed directly to hot exhaust gases, a layer of soot and other contaminants carried by the exhaust gases is rapidly deposited on the surfaces of the valve stem and the guide slot. Thus, after a short period of operation, the valve will become sluggish in operation and may break down completely.
It has also been found, with recycling valves of the type illustrated in the Nakajima et al. patent, that it is very difficult to adjust precisely the timing of the opening and closing of the valve, an adjustment which is of utmost importance for proper functioning of the recycling system. Another disadvantage is that if the valve body or the valve stem sticks in the valve housing due to icing, for example, proper operation of the throttle valve is also blocked, or at least made more difficult, because of the connecting linkage between the throttle valve and the recycling valve.